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Popular Digital Audio Formats
The term 'format' in this context refers to a group of different ways in which digital audio can be stored. In order to create or play back a given format, a piece of software called a codec is used. These codecs and formats all work on a particular system of encoding the sound into data. Digitial audio formats can be generally dropped into two categories, although the distinction can be somewhat blurry when examined closely. These categories are Lossy and Lossless. As part of the university project for which this Wiki was created, a small study was performed to determine general perception of audio quality with various audio formats. The results of that study can be found here: C Brown Listening Tests. It should also be noted that a given audio format is not necessarily tied to its file extension or container. Containers are the file types used to store data produced by a codec, 'written' in a particular format. For example, Apple's .m4a container is used both for ALAC (Lossless) as well as its AAC (lossy) formats, but the data within the file is quite different. This is not likely to be an exhaustive list. New formats are being developed all the time. Lossy Formats .mp3 - MPEG 2 Layer III. Uses a perceptual codec to remove the data which is least important for human hearing. .AAC - Advanced Audio Coding. Supposedly provides better sound quality than MP3 at similar file sizes and bitrates. Uses .m4a container for unprotected music, and .m4p container using DRM for protected music downloaded from the iTunes Store. .wma - Windows Media Audio. The WMA name (and container) actually covers four different codecs, including the standard WMA, WMA Professional more developed and efficient version of WMA, WMA Voice by Xbox Live and some other voice transmission services and WMA Lossless. .ogg - Vorbis. A format popular among users in various industries who do not wish to use other more highly patent-protected formats like .mp3. A large number of games companies use the Vorbis format for this reason - E.G. Mojang (Minecraft), Epic Games (Unreal Tournament 3), Frictional Games (Amnesia series), Blizzard (World of Warcraft), CCP (EVE Online). .Musepack/MPC. Works independently of any given container, so it can theoretically be packaged into any file type. ATRAC - Adaptive Transform Acoustic Coding. Developed by Sony and used mainly in their (now largely obsolete) MiniDisc format. Lossless Formats This first group of formats are in fact uncompressed. This means that the information about the sound is just recorded and played back 'as is' without the need for any further computing to reconstruct the compressed segments. The downside of this is that the files are necessarily much larger than their compressed counterparts. "Uncompressed" formats are included under the "lossless" banner as a matter of course, but in fact they differ slightly from the more specific definition of "lossless" formats. 'Lossless' formats like FLAC attempt to lose as little data as possible, but they compress the data in order to create a smaller resultant file. In practice uncompressed and lossless files should sound the same, and the integrity of the data is maintained, so they come under the same umbrella term. The main uncompressed digital audio format (stored as .wav and .aiff) is PCM. Uncompressed Formats: .wav (Waveform Audio File Format/WAVE). Wave is the Microsoft/IBM standard for audio storage on PCs. An implementation of PCM format. .AIFF (Audio Interchange File Format). Developed by Apple in 1988, used mostly on Apple computers and machines using iTunes. An implementation of PCM format. .Au - The standard format for Unix computer systems. Lossless Compressed Formats: .FLAC - Free Lossless Audio Codec. Arguably the most popular modern lossless compression format, it is an open-source (publically available and moddable) format. .m4a - ALAC (Apple Lossless Audio Codec). Similar to FLAC, but proprietary for Apple's hardware and software. .APE - Monkey's Audio. APE provides a higher compression ratio (a smaller file size) than FLAC and ALAC, with the unfortunate downside of making the files more difficult to play back in terms of computer power. This limits its potential use for mobile playback devices such as iPods. PCM and Perceptual Codecs Lossy formats such as .mp3, .ogg Vorbis, .aac do not make use of the PCM process. They use perceptual codecs, which use an equation called the Fourier Transform to translate and record sound information as frequency over time, as opposed to amplitude. This allows manipulation of how much data emphasis is applied to different frequency ranges, giving way to the lossy compression conventions which are based on removing data which is superfluous to normal human hearing. More information on this on the psychoacoustics wiki page.